This invention relates generally to computed tomography (CT) imaging and more particularly, to three dimensional artifact reduction in a CT system.
In at least one known CT imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighing algorithms which weight the collected data as a function of view angle and detector channel index. Specifically, prior to filtered back projection, the data is weighted according to a helical weighing factor, which is a function of both the gantry angle and detector angle. The helical weighting algorithms also scale the data according to a scaling factor, which is a function of the distance between the x-ray source and the object. The weighted and scaled data is then processed to generate CT numbers and to construct an image that corresponds to a two dimensional slice taken through the object.
It often is desirable to generate three-dimensional (3D) images or multi-planar reformatted images (herein referred to as 3-D images) of the object. Known algorithms for generating such images further process the helically weighted and scaled data. However, the 3D images typically include noticeable artifacts. Particularly, as a result of a heterogenous object being constantly translated during data acquisition, the attenuation distribution inside the scan plane changes continuously. To correct for this deviation from the basic tomographic reconstruction assumption, helical reconstruction is utilized to suppress the image artifacts. Recent developments of various computer graphic techniques applied to helical computed tomography, however, have discovered additional artifacts. These artifacts appear as periodical rings or grooves superimposed on the surface of the 3D image.
It would be desirable to provide an algorithm which facilitates the reduction of artifacts in 3D images due to the heterogenous object. It also would be desirable to provide such an algorithm which does not significantly increase the processing time.